[META] This commit should be followed up by removing the comments and assertions meant only to show that the change is correct.
LoadExternalBlockFile mainly acts on CChainState.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20749 | core#20749]] [13/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/e0dc3057277c9576ddbfb8541599db0149e08bb6
Depends on D11184